1. Field of the Invention
The present invention relates generally to the field of antenna systems for satellite communication and more particularly to small, portable, inexpensive, lightweight planar phased array antenna systems for transmission and reception of microwave signals.
2. Brief Description of the Prior Art
Satellite communication is getting more popular in the 1990's. Arrangements typically utilized for reception of Direct Broadcast Satellite (DBS) television signals include parabolic reflector dish antennas, used as front end antennas for residential homes and offices. Presently, these antennas are bulky, require a lot of mounting space, and are obtrusive looking, thus corrupting the harmony of our living environment. Lately however, portable and more user friendly satellite antenna systems are becoming popular, due to ever increasing demands for higher living standards by our highly mobile, dynamic society, and to the advances in modern science.
Typically, a modern portable communication system for DBS signals consists of a planar network of antenna elements serving to transfer signal energy from antenna circuits to space and, conversely, from space to antenna circuits.
The major difficulty in the design of antennas for reception of DBS television signals is obtaining sufficient reduction in size and weight, while having a gain high enough to be competitive with popular parabolic reflector dish antennas. A typical antenna for reception of DBS signals requires a carrier frequency of around 12 GHz (between 12.2 GHz and 12.7 GHz in the U.S.A.) and a gain of around 33.5 dBi. The typical planar antenna system is made as an array of such small antenna elements, in order to provide sufficient energy for satisfactory television pictures, with each antenna element being capable of receiving signals of around 12 GHz.
Recently, several types of planar antennas have been proposed for DBS reception. Some of the modern systems are printed-circuit and microstrip antenna systems, utilizing an array of antenna elements radiating circularly or linearly polarized waves, and sometimes having one or more waveguides. The major problems in designing a planar phased array antenna system for satellite communication are high manufacturing cost, high insertion loss of the combining network, and the difficulty in providing dual polarization performance with good isolation between the two polarization ports. The major costs of manufacturing a conventional printed phased array antenna system are the cost of microwave substrate materials and the cost of the etching process. Moreover, in presently used systems, even when using the best existing microwave substrate material, the printed array combining network insertion loss is still very high for the high gain satellite antenna.
Several published articles address the problems faced by a team of Japanese engineers in designing and redesigning their models of high efficiency flat antennas, using a multi-level, parallel plate, radial line and slot antenna concept. These models are described in the following articles: IEEE, Antennas and Propagation Society International Symposium 1993, Vol. 3, Jun. 28 through Jul. 2, 1993; IEEE, Antennas and Propagation Society International Symposium 1994, Vol. 2, Jun. 19 through 24, 1994; IEEE, Antennas and Propagation Society International Symposium 1994, Vol. 3, Jun. 19 through 24, 1994; and IEEE, Antennas and Propagation Society International Symposium 1995, Vol. 4, Jun. 18 through 23, 1994. The major drawbacks to this team's approach are the high cost of manufacturing a multi-level parallel plate system and the degraded performance of the system with reduced aperture size. In this approach, when the diameter of the aperture is reduced to less than 18 inches, the reflections from the end of the parallel plate radial line slot antenna degrade the antenna performance significantly, due to the great amount of energy left toward the end of the antenna. Further, as noted in these articles, at least two levels of parallel plates are required to achieve the dual polarization performance.
Although helpful, prior advances in the design of the phased array antenna systems are still unable to cover today's needs. The manufacturing cost for a system with dual parallel plates is high. It becomes prohibitive for multi-level, dual polarization flat antenna systems, some of which are described in Handbook of Microstrip Antennas, Vol. 2, 1989. According to this handbook, more than nine layers of printed circuits are required to achieve the dual polarization performance requirement. Moreover, in multi-level systems, the path length of the transmission line from the input port of the antenna array to each array element is very long, triggering high insertion loss of the array feed and high system noise.
The high ratio of the antenna gain over the noise temperature is another important requirement for a good receiving antenna system. The antenna aperture size, aperture efficiency and the loss of the array feed are the major factors used to determine the antenna gain. The low side lobe radiation pattern and the low resistive insertion loss of an antenna are the keys for achieving a low noise temperature of the antenna. However, there is a trade off between the low side lobe radiation pattern and the aperture efficiency. The lower the side lobe of the antenna is, the lower the noise temperature and the aperture efficiency will be. Therefore, achieving the lowest loss in the array feed circuitry design, especially the lowest resistive insertion loss, is the ultimate goal for all phased array antenna designers. Good isolation requirements can be achieved by designing antennas with a low cross polarization level relative to the co-polarization level and with good design of the associated circuitry between the two polarization ports.
Some modern planar phased array antenna systems use waveguides because they have the lowest insertion loss among all guided wave circuitry. Moreover, waveguides have the highest power handling capabilities, but they are expensive to build. All prior art antenna systems complying with the dual polarization requirement use at least two levels of the waveguide network, thus drastically increasing the manufacturing cost.
Other modern planar phased array antenna systems use air-striplines because they have the second lowest insertion loss and a good power handling performance, as well as a relatively low manufacturing cost.
There is a need for a high performance, phased array antenna system for receiving satellite communication signals, having a high ratio of the antenna gain over the noise temperature, low insertion loss of the combining network, a dual polarization performance with good isolation between the two polarization ports, and a low manufacturing cost.